Question

几天前我开始研究Android开发。我正在尝试从REST API获取数据，而我正在使用AsyncTask类来执行HTTP事务。问题是，我无法获取主活动的上下文，以便找到我的ListView并在onPostExecute（）中为其分配内容。

这是MainActivity.java：

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new DownloadMediaList(this).execute();
    }
}

这是DownloadMediaList.java：

public class DownloadMediaList extends AsyncTask<Void, Void, ArrayList<Media>> {

    ListView listView = null;
    Context mainContext = null;

    public DownloadMediaList(Context main){
        this.mainContext = main;
    }

    @Override
    protected void onPreExecute(){
        listView = (ListView) mainContext.this.findViewById(R.id.media_list);
    }

    // Operations that we do on a different thread.
    @Override
    protected ArrayList<Media> doInBackground(Void... params){
        // Set an ArrayList to store the medias.
        ArrayList<Media> mediaList = new ArrayList<Media>();

        // Call the REST API and get the request info and media list in a JSONObject.
        RESTFunctions restRequest = new RESTFunctions();
        JSONObject jsonMedia = restRequest.getMediaList();

        // Try catch to catch JSON exceptions.
        try {
            // Store the media list into a JSONArray.
            JSONArray mediaArray = jsonMedia.getJSONArray("media");

            // Create an instance of media to store every single media later.
            Media media = new Media();

            // Loop through the JSONArray and add each media to the ArrayList.
            for (int i=0; i<mediaArray.length();i++){
                media = new Media();
                JSONObject singleMedia = mediaArray.getJSONObject(i);
                media.setTitle(singleMedia.getString("titre"));
                media.setYear(singleMedia.getString("annee"));
                media.setLength(singleMedia.getString("duree"));
                mediaList.add(media);
            }
        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        // Return the ArrayList.
        return mediaList;
    }

    // Operations we do on the User Interface. Synced with the User Interface.
    @Override
    protected void onPostExecute(ArrayList<Media> mediaList){
        // Set TextViews in ListViews here
    }
}

这些类分为两个单独的文件。

这条线特别给我带来了麻烦：

listView = (ListView) mainContext.this.findViewById(R.id.media_list);

我做错了什么？它告诉我上下文无法解析为类型，即使我已经导入了android.content.Context。我试过实例化Context，但我也不能这样做。

Answer 1

Context没有观看次数，也没有findViewById方法 - 请尝试改为mainContext类型Activity。

Answer 2

替换

listView = (ListView) mainContext.this.findViewById(R.id.media_list);

与

listView = (ListView) mainContext.findViewById(R.id.media_list);

并且错误应该消失。

Answer 3

您可以将AsyncTask添加为活动中的内部类。

public class MainActivity extends Activity {
  private Context context;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    context=this;
    new DownloadMediaList(this).execute();
}

private class DownloadMediaList extends AsyncTask<Void, Void, ArrayList<Media>> {

ListView listView = null;




@Override
protected void onPreExecute(){
    listView = (ListView) context.findViewById(R.id.media_list);
}

// Operations that we do on a different thread.
@Override
protected ArrayList<Media> doInBackground(Void... params){
    // Set an ArrayList to store the medias.
    ArrayList<Media> mediaList = new ArrayList<Media>();

    // Call the REST API and get the request info and media list in a JSONObject.
    RESTFunctions restRequest = new RESTFunctions();
    JSONObject jsonMedia = restRequest.getMediaList();

    // Try catch to catch JSON exceptions.
    try {
        // Store the media list into a JSONArray.
        JSONArray mediaArray = jsonMedia.getJSONArray("media");

        // Create an instance of media to store every single media later.
        Media media = new Media();

        // Loop through the JSONArray and add each media to the ArrayList.
        for (int i=0; i<mediaArray.length();i++){
            media = new Media();
            JSONObject singleMedia = mediaArray.getJSONObject(i);
            media.setTitle(singleMedia.getString("titre"));
            media.setYear(singleMedia.getString("annee"));
            media.setLength(singleMedia.getString("duree"));
            mediaList.add(media);
        }
    } catch (JSONException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    // Return the ArrayList.
    return mediaList;
}

// Operations we do on the User Interface. Synced with the User Interface.
@Override
protected void onPostExecute(ArrayList<Media> mediaList){
    // Set TextViews in ListViews here
}
  }}

Answer 4

在您的活动课程中

    public interface TheInterface {
public void theMethod(ArrayList<Media> result);

 }
     }

然后

    DownloadMediaList task = new DownloadMediaList (MainAactivity.this,new TheInterface() {
             @Override
             public void theMethod(ArrayList<Media> result) {
                // result available here do whatever with list media
            }  
        });

然后在AsyncTask中

Context mainContext = null;
TheInterface mlistener;
public DownloadMediaList(Context main,TheInterface listener){
    this.mainContext = main;
    mlistener = listener; 
}

然后在onPostExecute

 @Override
protected void onPostExecute(ArrayList<Media> mediaList){
   if (mlistener != null) 
    {
         mlistener.theMethod(mediaList);
    }
}

编辑：

作为替代方案。

您可以在asycntask中定义接口，并让您的活动类实现接口。

上下文无法解析为某种类型

4 个答案: